网络操作系统是使网络上各计算机能方便而有效地共享网络资源，为用户提供所需的各种服务的软件和有关规程的集合。

 以下是对各种NOS产品的描述。

(53)由外层(Shell)和操作系统核心所构成，早期的产品的主要是用作网络文件服务器，并且采用了与TCP/IP不兼容的网络协议，从4.0版本以后开始引入分布目录服务、网络管理、远程通信等功能，并且开始支持TCP/IP。该操作系统是首次将容错引入NOS领域，其双机热备份功能是目前NOS领域中为优秀的容错方式。

(54)是UNIX操作系统的一个变种，是由Sun公司为自己生产的网络服务器设计的操作系统，早期的产品中有一部分称为SunOS，在2.6版本之后，和SunOS合并为新的7.0版本，并一直延续下去。由于Sun工作站、小型机在Internet服务方面的卓越成效，使其已成为Internet上的操作系统规范之一。

(55)是由Windows图形拥护接口子口基于OS\2的LAN Manager融合后的产物，分为Server和 Workstation两种版本。其组建的网络是典型的基于客户机/服务器计算机模式的局域网络，并且开始取代其他局域网操作系统，成为中小型企业网络的主流产品。该操作系统提供了功能强大的Backoffice服务器组件，为中小型企业(Small Business)提供企业网络建设。

(56)是为数不多的可以工作在微机上的UNIX产品，由芬兰赫尔辛基大学的Linus于1991年开始开发。同UNIX一样，是一个多用户、多任务操作系统，由内核、壳(Shell)与应用程序三层构成；可以在普通PC上实现中小型服务器的功能，同时由于IBM等大型服务器厂商宣布对其的支持，使得该操作系统的发展充满光明。

(57)是典型的对等局域网络操作系统，在小型局域网上的主要的应用是在小组内共享数据和打印机等网络资源，不需要专门的中央服务器。公开发行较早，目前已经退出可操作系统流通领域，与其后推出的后续产品有所不同的是，它是完全的对等局域网操作系统，而其后续产品中只是包含了对等网络的功能。

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While the Internet is inherently insecure, businesses still need to preserve the privacy of data as it travels over the network. To help do that, the Internet Engineering Task Force an international group of network designers, operators, vendors, and researchers concerned with the evolution of(71)has developed a suite of(72)called Internet Protocol Security(IPSec).

 IPSec creates a standard platform. to develop secure networks and electronic(73)between two machines. Secure tunneling via IPSec creates(74)connections in a network through which data packets can move. It creates these tunnels between remote users and within a local network. It also(75)each data packet in a new packet that contains the information neccessary to set up, maintain and tear down the tunnel when it's no longer needed.

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One of the difficulties in building an SQL-like query lange for the Web is the absence of a database schema for this huge, heterogeneous repository of information. However, if we are interested in HTML documents only, we can construct a virtual(66)from the implicit structure of these files. Thus, at the highest level of(67), every such document is identified by its Uniform. Resource Locator(URL), has a title and a text Also, Web servers provide some additional information such as the type, length, and the last modification date of a document. So, for data mining purposes, we can consider the site of all HTML documents as arelation:

 Document(url,(68), text, type, length, modify)

 Where all the(69)are character strings. In this framework, anindividual document is identified with a(70)in this relation. Of course, if some optional information is missing from the HTML document, the associate fields will de left blank, but this is not uncommon in any database.

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